1. Field of the Invention
The present invention relates to an anion-selective, sensitive film containing an anion sensitive substance supported on a polymeric film, which is suitable for measuring inorganic anions, particularly chloride ions contained in body fluids, and to an anion-selective electrode using such a sensitive film. The invention also relates to an anion-selective field effect transistor, a biochemical analysis apparatus provided with such a transistor, and to a method of measuring anion concentration by employing such an anion-selective electrode or transistor.
2. Related Art Statement
It is known to use a solid film type electrode comprising silver chloride and silver sulfide as an anion-selective electrode for measuring anions contained in body fluids. It is also known to use a film type electrode comprising an ion exchanger material as sensitive film supported on a polymeric film such as polyvinyl chloride film. The former solid film type electrode has a drawback that its performance is rather low due to a large adverse effect of sulfide ions or halide ions such as bromide ions. On the other hand, the latter electrode has a short-coming that there might be an error of measurement due to adverse influences of lipophilic ions which are apt to deposit on the sensitive film.
Therefore, efforts have been made for an improvement of ion-selectivity of an electrode having a high polymer film type support. For instance, Japanese Patent Application Kokai (Laid-open) No. 56-63246 proposes to use a tetraalkyl ammonium chloride, which is an example of quaternary ammonium salts, as a sensitive substance, and also to use a straight chain alcohol as plasticizer.
Japanese Patent Application Kokai (Laid-open) No. 59-137851 discloses the use of a dimethyl dioctadecyl ammonium salt for an improvement of the selectivity for chloride ions and for reducing an adverse influence of lipophilic ions in the measurement of the aimed ions, particularly chloride ions.
In Mikrochimica Act (Wien), 1984 III, 1-16 (1984), it is mentioned that tetradodecyl ammonium salts, which belong to the class of tetraalkyl type, ammonium salts, have an improved selectivity for hydrophilic hydrogen carbonate ions.
However, it is noted that known chloride ion-selective electrodes, which contain the above-mentioned known sensitive substances, have a rather low selectivity for chloride ion over lipophilic ions but also low selectivity for chloride ion over hydrophilic ions. Thus, up to now, it has been difficult to improve not only the selectivity for lipophilic ions but also the selectivity for hydrophilic ions where use was made of the known sensitive substances. For the reasons mentioned above, there have occurred a number of problems about the characteristic properties of electrodes and the measurement accuracy when a measurement of a sample such as blood serum is made by employing any of the known electrodes.
Furthermore, an anion-selective electrode according to the prior art does not have a long effective time, so that the measurement accuracy would decrease after a relatively short period of time when the electrode is used for the measurement of a large number of controlled blood serum samples.